Integrated circuit devices typically include various active and passive circuit elements which have been integrated into or onto a substrate of semiconductor material, often referred to as a die. The die may, in turn, be utilized in a package, which often includes a ceramic or plastic substrate supporting the die although other materials may be used. These packages are usually attached to a printed circuit board, often by conductors arranged on the exterior of the package. In this manner, an electronic system can be assembled by connecting various integrated circuit devices to a printed circuit board. In some applications, a die may be utilized in a system without a package substrate supporting the die.
To increase space utilization, two or more integrated circuit dies may be attached to a printed circuit board in a stacked arrangement. The dies may be interconnected in a die-to-die stacked arrangement to form a package. Alternatively, each die may be placed in a package and the two packages may be stacked in a package-to-package arrangement. FIG. 1 shows an exploded view of one such package-to-package stack type integrated circuit device which is indicated generally at 50. The device 50 includes a first integrated circuit package 52, and a second integrated circuit package 54 which are physically and electrically connected together using a foldable, flexible substrate 56 of the package 52. In addition to package to package interconnections, folded substrates may be used to provide die-to-die or die-to-package substrate interconnections.
The foldable substrate 56 includes a first substrate interconnect area 60 on which an integrated circuit die 62 is mechanically and electrically connected by a plurality of solder bumps 64. Similarly, the integrated circuit package 54 includes a package substrate 66 to which an integrated circuit die 68 is mechanically and electrically connected by a plurality of solder bumps 70. Other electrical connectors including wires 72 may be used in place of or in addition to the solder bumps 64, 70 to connect the dies to the substrates.
The package 54 includes a third die 74 stacked on the die 68 with a spacer layer 76 therebetween. Additional wires 72 connect the die 74 to the package substrate 66. The package substrates 56, 66 may have both internal and exterior conductors which are electrically connected to the solder bumps 64, 70, wires 72 or to contact pads on the dies 62, 68, 74.
The dies 62, 68 and 74 may be encapsulated in polymers such as an epoxy layer 78 depicted for the die 64. The inputs and outputs of the stack 50 may be electrically connected to a printed circuit board using connection pins, solder bumps or other connection terminals represented at 80 extending from the substrate area 60 of the foldable substrate 56.
The substrate 56 includes a foldable, flexible area 82 which permits a second substrate interconnect area 84 of the substrate 56 to be folded over and attached to the top of the encapsulation layer 78 using a layer 86 of adhesive. The substrate 56 has flexible conductor traces deposited on the foldable area 82 which electrically connect the substrate interconnect area 84 to the substrate interconnect area 60. The inputs and outputs of the package 54 may be mechanically and electrically connected to the substrate area 84 of the foldable substrate 56 using appropriate connection terminals represented at 90 extending from the substrate 66 of the package 54.
In addition to folded substrates, integrated circuits may be stacked with interposers. Other integrated circuit stacks may be formed without folded substrates or interposers.
FIG. 2 illustrates a known technique for bonding connection terminals or other conductors to a substrate which may be a package substrate, a semiconductor substrate or other substrate utilized in integrated circuits and integrated circuit packaging. In this example, several substrates 91a, 91b . . . 91n are placed into a bonding tray 92 which is typically formed of injection molded plastic. For example, the bonding tray 92 may be formed of thermoplastic polyimide with a high concentration of carbon fiber and glass fiber. The bonding tray 92 typically has formed within it a plurality of pockets 94, each of which receives an individual substrate 91a, 91b . . . 91n. 
Each pocket 94 relatively precisely positions the substrate 91a, 91b . . . 91n relative to the bonding tray 92. In this example, a plurality of solder balls 90 (FIGS. 2, 2a) are positioned on corresponding lands 96 of each substrate 91a, 91b . . . 91n in a ball grid array. The conductors may be placed on the lands 96 of the substrates 91a, 91b . . . 91n manually or using automated equipment. Often, the lands 96 are coated with a suitable flux material prior to depositing the solder balls 90 on to the lands 96.
The solder balls 90 may be bonded to the lands 96 of the associated substrate 91a, 91b . . . 91n by the application of sufficient heat to cause the solder of the balls 90 to flow. Such heat is typically applied by placing a bonding tray 94 containing the substrates 91a, 91b . . . 91n with solder balls 90 in an oven. One such oven is an infrared solder reflow oven which is operated at a temperature sufficient to bond the balls 90 to the lands 96 of the associated substrate 91a, 91b . . . 91n. In this manner, a complete or partial integrated circuit device may be formed.